I had that with my 1st pregnancy... it was b/c I had very low iron.
Its perfectly normal dont worry :-)
Yes thats normal in the second trimester. It is usually caused by blood pressure etc but since yours is fine then theres nothing to be concerned about. Im 30 weeks with my second and have started seeing the stars aswell. But mine at the moment is caused by my blood pressure and sugar because I was really sick and have struggled to put on weight and keep any food down. With my son I saw them around 20 weeks due to my excess weight gain. When getting up, don't go quick. Just pase yourself. You will make yourself dizzy. When laying down, sit up and put your feet on the floor and sit down for a bit longer then usual and then slowly stand up. You have increased blood flow through your body.